Another off-the-cuff use of scarcity last night was for burning lucidity when under pressure.
The situation was that there were reanimated being attracted by a military communications truck. For weeks they'd been gathering at the truck (powered by solar) and there were thousands of them milling around. We have a massive rock festival in Reading, so we're all very aware of what that's like. The team headed over to see why the zombies were congregating. They climbed from roof to roof (quite easy in suburban Reading as the gaps between rooftops are normally just a few feet) to get close the hospital which was the epicentre.
I wanted to give them a risk of voluntarily surrounding themselves with thousands of reanimates - one fall from the roof would mean game over. That's stressful. To show that, I started everyone on a D12 and a 1 meant they would lose 1 lucidity. The more terrifying it got, the smaller the die they'd roll. I gave plenty of warning and option to turn back.
Where this came in really useful was that Aggro (sniper player) saw that there were military reanimates running the show and they were armed. I told him in private. He then had the choice: tell the rest of the team (and force a die reduction and roll) or keep it secret. He chose to keep it secret, so rolled himself. A similar case happened a bit later with another player. Half the group knew, the other half probably would have guessed but the characters didn't know. They found out when running into close combat.
I liked this system. Some lucidity was lost and the die reduction go to the point (d8) where things were looking touch-and-go. Byrn started with the lowest lucidity and lost a bit more (but used AP at the end of the session to buy back some) - I think he got down to the magic 5.
All in all, I liked it as an application of the scarcity system but I will be using sparingly. It was an extreme circumstance that caused them to roll.